Prepare for termbox/windows build

`TAGS=termbox make` (or `go build -tags termbox`)

src/constants_unix.go

@@ -0,0 +1,8 @@
+// +build !windows
+
+package fzf
+
+const (
+	// Reader
+	defaultCommand = `find . -path '*/\.*' -prune -o -type f -print -o -type l -print 2> /dev/null | sed s/^..//`
+)